Recovery of hydrocarbons from a subterranean reservoir by heating



United States Patent 3,205,944 RECOVERY OF HYDROCARBONS FROM A SUBTEEAN RESERVOIR BY HEATING Dean K. Walton, Dallas, Tex., assignor to Socony Mobil Oil Company, Inc, a corporation of New York No Drawing. Filed June 14, 1963, Ser. No. 237,756 11 Claims. (Cl. 16611) This invention relates to the recovery of hydrocarbons from a subterranean formation containing hydrocarbon material and relates more particularly to the recovery of such hydrocarbons by a procedure involving heating of the formation.

Various methods involving heating have been proposed for the recovery of hydrocarbons from subterranean formations containing a hydrocarbon material. With heating of the hydrocarbon material within the formation, the viscosity of the material is reduced or the chemical composition of the material is changed to form a material which has a lower viscosity. In either case, the hydrocarbon material in the formation is able to flow more readily through the formation and its recovery from a production, or output, well is thereby facilitated. One of these methods involves combustion of a portion of the hydrocarbon material within the formation. In this method, an oxidizing gas is passed into the formation through an input, or injection, well and the hydrocarbon material within the formation is ignited by suitable means. The zone of combustion, or combustion front, produced by ignition migrates through the formation and the hydrocarbons released from the formation by the migrating combustion front are driven through the formation into the direction of the output, or production, well. The hydrocarbons enter the output well and they are removed therefrom and brought to the surface of the earth. While this method, termed the in-situ combustion method, is satisfactory from the standpoint of the results desired, it is subject to certain drawbacks. Primarily, the in-situ combustion method suffers from the drawback that an appreciable portion of the hydrocarbon material in the formation is consumed by the migrating combustion front with consequent decrease in the proportion of hydrocarbons that can be recovered. Associated therewith is the drawback that the excessively high temperatures attained by combustion of the hydrocarbon material require close control in order to avoid damage or destruction to equipment in both the input and the output wells. Methods of effecting heating in the formation other than by in-situ combustion are also available. For example, fluids heated at the surface of the earth can be passed into the formation through an input well. However, these methods suffer from the drawback that they require the utilization of a fuel, with concomitant expense, to heat the fluid passed into the formation.

It is an object of this invention to provide a method for heating a subterranean formation containing hydrocarbon material for recovery of hydrocarbons from the formation.

It is another object of this invention to reduce the cost of heating a subterranean formation containing hydrocarbon material.

It is another object of this invention to avoid excessive heating of wells penetrating a subterranean formation containing hydrocarbon material for recovery of hydrocarbons from the formation.

It is another object of this invention to recover hydrocarbons from a subterranean formation by a low temperature heating procedure.

It is another object of this invention to increase the proportion of hydrocarbons recovered from a subterranean formation by a heating procedure which does not the hydrocarbon material within the formation.

3,205,944 Patented Sept. 14, 1965 require the passage of a previously heated fluid through the formation.

These and further objects of the invention will become apparent from the following detailed description.

In accordance with the invention, there is provided a procedure which involves as its essential steps passing an oxidizing gas through a subterranean formation containing hydrocarbon material to effect auto-oxidation of the hydrocarbon material and thereby heat' the hydrocarbon material, controlling the composition of the oxidizing gas so as to maintain the temperature within the formation below the ignition temperature of the hydrocarbon material and thereby prevent combustion of the hydrocarbon material, and thereafter passing an inert driving fluid through the formation to drive hydrocarbons to an output Well. In a more specific aspect, the procedure involves the steps of (a) passing an oxidizing gas through a subterranean formation containing hydrocarbon material between an input well and an output well, (b) maintaining the passage of the oxidizing gas through the formation between the input well and the output well whereby auto-oxidation of the hydrocarbon material in the formation occurs as the result of the passage of the oxidizing gas, (6) reducing the oxygen content of the oxidizing gas passed into the input well through the formation whenever the carbon dioxide content of the effluent gas from the formation into the output Well exceeds about 3 percent by volume so as to maintain the carbon dioxide content of the effluent gas not in excess of this amount, (d) repeating steps (a), (b), and (0) until the average temperature of the hydrocarbon material'in the formation has been increased as a result of the auto-oxidation at least F. above the average temperature prior to passage of the oxidizing gas, and (e) thereafter passing through the formation between an input well and an output well an inert fluid to drive heated fluid hydrocarbons from the formation into the output well.

It has been observed that passage of an oxidizing gas through a subterranean formation containing hydrocarbon material can effect auto-oxidation of the hydrocarbon material within the formation. This auto-oxidation occurs at a relatively low rate and the exothermic heat of reaction is consequently released slowly. For some hydrocarbon materials, at least, the rate of auto-oxidation at lower pressures of the oxygen in the oxidizing gas is directly proportional to the pressure of the oxygen. At higher pressures, however, the rate of auto-oxidation is practically independent of the pressure of the oxygen. With prolonged passage of the oxidizing gas through the formation, the auto-oxidation occurs to a significant extent and effects appreciable increase in the temperature of the formation. Moreover, the rate at which the autooxidation occurs progressively increases with increase in the temperature of the formation. Accordingly, the rate of increase in the temperature of the formation with continued passage of the oxidizing gas becomes progressively greater with time. Thus, in consequence of the passage of the oxidizing gas through the formation, the average temperature of the formation between the input well and the output well can be increased to a desired extent.

As the result of the auto-oxidation, the temperature attained Within the formation can be the ignition temperature of the hydrocarbon material within the formation. Thus, in accordance with the invention, the auto-oxidation reaction is controlled in order to prevent combustion of Control, as indicated, is effected by reducing the oxygen content of the oxidizing gas passed through the formation.

As the oxidizing medium passes through the formation from the input well to the output well and auto-oxidation of the hydrocarbon material occurs, carbon dioxide is produced. Concomitantly with the production of carbon dioxide, the oxygen content of the oxidizing medium is decreased. Accordingly, a measure of the rate at which auto-oxidation of the hydrocarbon material within the formation is occurring is the carbon dioxide content, or the oxygen content, of the eflluent gas passing into the output well from the formation. Since the rate at which the auto-oxidation occurs increases with increase in the temperature of the hydrocarbon material in the formation, the rate of auto-oxidation is also a measure of the temperature of the formation. Thus, the carbon dioxide content of the efliuent gas is a measure of the temperature of the hydrocarbon material in the formation. When the carbon dioxide content of the efliuent gas from the output well exceeds about 3 percent by volume, the temperatures within the formation are beginning to approach the ignition temperature of the hydrocarbon material in the formation. At this point, the oxygen content of the oxidizing gas is reduced.

Determination of the carbon dioxide content of the efliuent gas may be by any desired means. For example, the eflluent gas may be analyzed for its carbon dioxide content by employing an Orsat apparatus. Analysis may be also made employing apparatus that detect changes in the physical properties of the eflluent gas with changes in the carbon dioxide content. Additionally, analysis may be made continuously or intermittently.

To reduct the oxygen content of the oxidizing gas passed through the formation from the input well, an inert fluid is passed through the formation. By inert fluid is meant any gas or liquid which will not effect oxidation of the hydrocarbon material within the formation. Suitable fluids include nitrogen, carbon dioxide, a gas containing carbon dioxide such as flue gas, methane, or a gas containing methane such as natural gas. Preferably, however, the inert fluid is water. Mixtures of inert fluids may also be employed, if desired.

The inert fluid may be passed throughthe formation in admixture with the oxidizing gas. On the other hand, passage of the oxidizing gas may be discontinued temporarily and the inert fluid passed through the formation without admixture with the oxidizing gas. Further, the inert fluid and oxidizing gas may be passed alternately into the formation.

With passage of the inert fluid through the formation, the temperature within the formation is reduced. This reduction in the temperature of the formation is effected as a result of transfer of heat to the inert fluid. Where the inert fluid is a liquid, such as water, part of the heat transferred to the fluid is utilized as heat of vaporization of the fluid. The heated fluid, as it passes through the formation to the output well, gives up part of its heat to the color portions of the formation. Thus, not only is the average temperature within the formation reduced as a result of the passage of the inert fluid, but a more uniform temperature throughout the formation is attained. Stated otherwise, a passage of the inert fluid reduces the temperature of the formation where comparatively high temperatures exist and increases the temperature of the formation where comparatively low temperatures exist. The amount of inert fluid passed through the formation will be the amount required to maintain the carbon dioxide content of the eflluent gas not greater than about 3 percent by volume. On the other hand, the amount employed should not be such that the auto-oxidation is reduced to an excessively low rate. Preferably, the amount of inert fluid passed through the formation should not be in excess of the amount to. reduce the carbon dioxide content of the eflluent gas below about 0.5 percent by volume.

The steps of passing the oxidizing gas through the formation until the carbon dioxide content of the eflluent gas from the production well becomes as high as about 3 percent by volume and passing the inert fluid through is water.

the formation to reduce the carbon dioxide content of the eflluent gas are continued until the formation between the input well and the output well has been heated. The passage of the oxidizing gas and the inert fluid may be intermittent. However, with intermittent operation, the temperature of the formation decreases during the time the passage of the oxidizing gas is discontinued. It is therefore preferred to pass the oxidizing gas and the inert fluid continuously through the formation to effect the desired heating without unnecessary delay.

With increase in the temperature of the hydrocarbon material within the formation, the viscosity of fluid hydrocarbon material will be decreased. With lower viscosity, the rate at which the hydrocarbon material can be produced from the formation to the output well, with other conditions being equal, will be increased. The rate at which the hydrocarbon material can be passed through the formation to the output well can be expressed as a function of the mobility ratio of the hydrocarbon material in the formation and the inert driving fluid employed in the last step of the procedure. The mobility ratio is defined as the quotient of the product of the relative permeability of the reservoir to the inert driving fluid and the viscosity of the hydrocarbons and the product of the effective permeability of the formation to the hydrocarbons and the viscosity of the inert driving fluid. The mobility ratio is expressed by the following formula:

wl o where k is the relative permeability of the reservoir to the inert driving fluid,

k is the effective permeability of the reservoir to the hydrocarbons,

,u is the viscosity of the inert driving fluid, and

,u is the viscosity of the hydrocarbons.

Preferably, the formation should be heated by the autooxidation to an average temperature such that the mobility ratio of the fluids is below 10 and preferably below 3. With the attainment of such mobility ratio, effective recovery of the hydrocarbon material, as compared to re covery in the absence of the heating procedure of the inyention, can be achieved.

The increase in the average temperature within the formation required to achieve the desired mobility ratio of the fluids will depend upon the viscosity characteristics of the hydrocarbons and the average temperature within the formation prior to the heating procedure. In any case, effective decreases in the mobility ratio can usually be attained when the formation is heated to an average temperature which is at least F. greater than the average temperature prior to the heating procedure. Preferably, however, the formation should be heated to a temperature which is at least 250 F. greater than its temperature prior to the heating procedure. In any case, the formation should not be heated to a temperature which is more than 300 F. greater than its temperature prior .to the heating procedure.

Following attainment of the desired mobility ratio of the fluids and average temperature within the formation, passage of the oxidizing gas through the formation is discontinued. Thereafter, an inert driving fluid is passed through the formation from an input well to an output well to assist in recovery of the hydrocarbon material. The input well and the output well may be the same input well and output well employed in passing the oxidizing gas through the formation. However, another input well and another output well may be employed. If desired, the output well employed for passage of the oxidizing gas may be employed as the input well for the inert driving fluid and vice versa. Preferably the inert driving fluid On the other hand, a driving fluid such as a gas containing carbon dioxide, forexample flue gas, or

natural gas, nitrogen, or other fluid which will not effect oxidation of the hydrocarbon material within the formation may be employed. With passage of the inert driving fluid through the formation, the fluid hydrocarbons within the formation are driven through the formation to the output well from which they can be recovered by conventional means.

While the procedure of the invention has been described above in connection with the use of a single input well and a single output well, it will be understood that a plurality of input wells or a plurality of output wells may be employed. Thus, for example, the procedure of the invention may be employed in connection with a 5-spot pattern. In this pattern, four wells located at the corners of a square are employed as output wells and a well located centrally of these four output wells is employed as an input well. Additionally, a line of input wells may be employed along with a line of output wells. It is also possible to employ a single well in which the oxdizing gas and inert fluid are passed into the formation at one height and the eflluent gas and hydrocarbon material pass from the formation at another height. Any other conventional well pattern may also be employed.

In the practice of the invention, it is preferred that the oxidizing gas be passed into the formation from the input well at an elevated pressure. As stated, the rate of autooxidation of some hydrocarbon materials at lower pressures of the oxygen in the oxidizing gas is directly proportional to the pressure of the oxygen, but at higher pressures is practically independent of the pressure of the oxygen. Thus, the oxidizing gas is passed into the formation at a pressure of at least 8 atmospheres to take advantage of the reduced dependence of the oxygen pressure on the rate of auto-oxidation.

The oxidizing gas employed in the practice of the procedure may be any gas containing oxygen. Preferably, air is employed. However, if desired, other oxidizing gases may be employed. For example, oxygen may be employed. Further, oxygen enriched air may be employed. Auto-oxidation of the hydrocarbon material within the formation may be obtained employing air containing another gas, such as nitrogen, to reduce the amount of oxygen. Where air containing another gas to reduce the amount of oxygen is employed, the amount of inert fluid required in the second step to maintain the carbon dioxide content of the etfluent gas not in excess of about 3 percent by volume will be reduced. On the other hand, the use of air containing another gas to reduce the amount of oxygen will decrease the pressure of the oxygen and thus decrease the rate of auto-oxidation with consequent increase in the time required for the formation to attain the desired temperature.

The procedure of the invention may be carried out in any type of subterranean formation containing hydrocarbon material or a material providing a source of hydrocarbons. Thus, the procedure may be carried out in a formation containing petroleum, in an oil shale forma tion, or in a tar sand formation. Further, the formation may be preliminarily treated, if desired, or necessary, for the purposes of establishing, or increasing, permeability. Thus, for example, the formation preliminarily may be acidized or may be fractured.

By the procedure of the invention, recovery of hydrocarbons is effected without excessive heating of the formation. While some hydrocarbon material is consumed as a result of the auto-oxidation, the amount thus consumed is insignificant. Further, heating is effected without the necessity of utilizing a fuel to heat a fluid medium to be passed into the formation. Moreover, the temperatures attained within the formation are well below those that would damage or destroy equipment in the input or output wells.

Having thus described my invention, it will be understood that such description has been given by way of illustration and example and not by way of limitation, referonce for the latter purpose being bad to the appended claims.

I claim:

1. A procedure for recovering hydrocarbons from a subterranean formation containing hydrocarbon material comprising:

(a) passing an oxidizing gas through said formation from an input well to an output well to effect autooxidation of said hydrocarbon material in said formation and thereby heat said hydrocarbon material,

(b) maintaining said passage of oxidizing gas through said format-ion until said hydrocarbon material in said formation has been heated as a result of said auto-oxidation to an average temperature at least F. above its average temperature prior to the passage of said oxidizing gas,

(c) reducing the oxygen content of said oxidizing gas whenever the carbon dioxide content of the efiluent gas from said formation to said output well exceeds about 3 percent by volume so as to maintain said carbon dioxide content not in excess of about 3 percent by volume, and

(d) thereafter passing an inert driving fluid through said formation from an input well to an output well to drive heated fluid hydrocarbon in said formation to said latter output well.

2. The procedure of claim 1 wherein the oxygen content of said oxidizing gas is reduced so as to maintain the carbon dioxide content of the effluent gas from said formation to said output well below about 3 percent by volume but not below 0.5 percent by volume.

3. A procedure for recovering hydrocarbons from a subterranean formation containing hydrocarbon material comprising:

(a) passing an oxidizing gas through said formation from an input well to an output well,

(b) maintaining said passage of said oxidizing gas through said formation to effect auto-oxidation of said hydrocarbon material in said formation and thereby heat said hydrocarbon material,

(c) reducing the oxygen content of said oxidizing gas when the carbon dioxide content of the efiluent gas from said formation to said output well exceeds about 3 percent by volume,

(d) repeating said steps of maintaining passage of said oxidizing gas through said formation and reducing the oxygen content of said oxidizing gas when said carbon dioxide content of said eflluent gas from said formation to said output well exceeds about 3 percent by volume until the average temperature of said hydrocarbon material in said formation has been increased at least 100 F. above its average temperature prior to the passage of said oxidizing gas, and

(e) thereafter passing an inert driving fluid through said formation between an input well and an output well to drive heated fluid hydrocarbons in said formation to said latter output well.

4. The procedure of claim 3 wherein said oxidizing gas is passed through said formation from said input well at a pressure of at least 8 atmospheres.

5. The procedure of claim 3 where-in said oxidizing gas is passed through said formation from said input Well until the average temperature of said hydrocarbon material in said formation has been increased at least 250 F. but not more than 300 F. above its average temperature prior to the passage of said oxidizing gas.

6. The procedure of claim 3 wherein the oxygen content of said oxidizing gas is reduced by adding an inert fluid to said oxidizing gas.

7. The procedure of claim 3 wherein the oxygen content of said oxidizing gas is reduced by adding water to said oxidizing gas.

8. The procedure of claim 3 wherein said heated fluid hydrocarbons in said formation are driven to said output well by passing water through said formation from said input well to said output well.

9. The procedure of claim 3 wherein said oxidizing gas is air.

.10. A procedure for recovering hydrocarbons from a subterranean formation containing hydrocarbon material comprising:

(a) passing an oxidizing gas through said formation from an input well to an output well to eflect autooxidation of said hydrocarbon material in said formation and thereby heat said hydrocarbon material to an elevated temperature less than the ignition temperature of the hydrocarbon material in said formation and thereupon,

(d) reducing the oxygen content of said oxidizing gas passing through the formation sufficiently to prevent ignition and combustion of said hydrocarbon material whenever the temperature of said hydrocarbon material in said formation begins to approach the ignition temperature of said hydrocarbon material,

(c) maintaining thepassage of oxidizing gas through said formation until the formation is heated by autooxidation to an average temperature such that the mobility ratio, which is the ratio of the mobility of the driving fluid to the mobility of the driven fluid therein, is below 10, and

(d) thereafter passing an inert driving fluid through said formation from an input well to an output well to drive heated fluid hydrocarbons within said formation to said latter out-put well.

11. A procedure for recovering hydrocarbons froma subterranean formation containing hydrocarbon material comprising:

(a) passing an oxidizing gas through said formation '8 from an input well to an output well to effectautooxidation of said hydrocarbon mat'erialin saidv formation and thereby heat said hydrocarbon material to an elevated temperature less than the ignition temperature of the hydrocarbon material insaid formation and thereupon,

(b) reducing the oxygen con-tent of saidoxidizing gas passing through the formation sufficiently to prevent ignition and combustion of said hydrocarbon materia'l whenever the temperature of said hydrocarbon material in said formation begins to approach the ignition temperature of said hydrocarbon material,

(c) maintaining the passage of oxidizing gasthrough said formation until said hydrocarbon material in said formation has been heated as a'result of said auto-oxidation to an average temperature at least 100 F. above its average temperature prior to the passage of said oxidizing gas, and v (d) thereafter passing an inert driving fluid through said formation from an input well to an output well to drive heated fluid hydrocarbons within said forma- .tion to said latter output well.

References Cited by the Examiner UNITED STATES PATENTS 2,780,450 2/57 Ljungstom 166-11 3,026,937 3/62 Simm 16611 X 3,036,632 5/62 Koch et a1 166-1.1 3,097,690 7/63 Terwilliger et al 16611 3,110,345 11/63 Reed et a1 166-11 3,111,986 11/63 Kuhn 166-11 BENJAMIN HERSH, Primary Examiner. 

10. A PROCEDURE FOR RECOVERING HYDROCARBONS FROM A SUBTERRANEAN FORMATION CONTAINING HYDROCARBON MATERIAL COMPRISING: (A) PASSING AN OXIDIZING GAS THROUGH AID FORMAION FROM AN INPUT WELL TO AN OUTPUT WELL TO EFFECT AUTOOXIDATION OF SAID HYDROCARBON MATERIAL IN SAID FORMATION AND THEREBY HEAT SAID HYDROCARBON MATERIAL TO AN ELEVATED TEMPERATURE LESS THAN THE IGNITION TEMPERATURE OF THE HYDROCARBON MATERIAL IN SAID FORMATION AND THEREUPON, (B) REDUCING THE OXYGEN CONTENT OF SAID OXIDIZING GAS PASSING THROUGH THE FORMATION SUFFICIENTLY TO PREVENT IGNITION AND COMBUSTION OF SAID HYDROCARBON MATERIAL WHENEVER THE TEMPERATURE OF SAID HYDROCARBON MATERIAL IN SAID FORMATION BEGINS TO APPROACH THE IGNITION TEMPERATURE OF SAID HYDROCARBON MATERIAL, (C) MAINTAINING THE PASSAGE OF OXIDIZING GAS THROUGH SAID FORMATION UNTIL THE FORMATION IS HEATED BY AUTOOXIDATION TO AN AVERAGE TEMPERATURE SUCH THAT THE MOBILITY RATIO, WHICH IS THE RATIO OF THE MOBILITY OF THE DRIVING FLUID TO THE MOBILITY OF THE DRIVEN FLUID THEREIN IS BELOW 10 AND (D) THEREAFTER PASSING AN INERT DRIVING FLUID THROUGH SAID FORMATION FROM AN INPUT WELL TO AN OUTPUT WEL TO DRIVE HEATED FLUID HYDROCARBONS WITHIN SAID FORMATION TO SAID LATTER OUTPUT WELL. 